Braley later apologized and his campaign quickly shot out a series of releases touting the candidate's rural Iowa background.

Warren, a freshman senator from Massachusetts, and the progressive group also announced they are backing Rick Weiland in his candidacy for an open U.S. Senate seat in South Dakota.

"Two of my Democratic colleagues in the Senate, Tom Harkin of Iowa and Tim Johnson of South Dakota, are retiring this year. Karl Rove and the Koch Brothers are already spending huge amounts of money to pick up those seats," Warren said in a release.

Weiland, the only Democrat in the race, threw his hat in the ring after Democratic Sen. Tim Johnson announced last year he would not seek reelection.

Johnson's announcement and a string of other Senate Democratic retirements have further stoked Republican hopes of picking up enough Senate seats in November's midterm election to force Democrats in the minority in the upper chamber.